Accelerometers are used in a wide range of applications such as inertial navigation systems, safe-and-arming weapons, geophysical exploration, and automotive crash sensing. The efficacy of devices utilizing accelerometers can be greatly enhanced by miniaturization. Thus, silicon micromachined accelerometers have been developed beginning with a single cantilever structure with a silicon mass and piezoresistive sensor as described in Roylance, L. M., and J. B. Angell, "A batch-fabricated silicon accelerometer", IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, 1979, p. 1911.
In another method to measure the acceleration detected by an accelerometer, as disclosed in U.S. Pat. No. 4,711,128 to Boura, a pendulum type mass with capacitive plates is suspended between two fixed capacitive plates. The mass is kept in a neutral position by electrostatically balancing the forces on the mass. The required electrostatic return force to maintain the balance is monitored to provide a measure of the detected acceleration. Variations of this method of using capacitors to produce electrostatic return forces are disclosed in U.S. Pat. No. 4,393,710 to Bernard and U.S. Pat. No. 4,566,328 to Bernard et al.
A problem which arises for all accelerometers used in applications which require ultra-high reliability, such as safe-and-arming and automobile air-bag devices, is to determine whether the accelerometer is functioning and functioning accurately. Often it is imperative that failure of accelerometer-based sensors be known as quickly as possible. For instance, in geophysical exploration applications, large numbers of sensors are chained together and any drop-outs in the array due to non-functional sensors would produce inaccurate results and/or degrade resolution. Further, in inertial guidance systems and geophysical applications, the operating environment can be extremely hostile, resulting in high failure rates. In other applications, such as fusing projectiles and air-bags, it is critical to determine sensor defects which may cause unexpected detonation or failure to detonate when required.
In the prior art, piezoresistive sensors have required that testing and calibration be done using an external forcing function. Typically, an external force is applied and the results are noted for testing and measured to provide a calibrated output. This approach has the disadvantage that the sensor must be subjected to the same forcing function for calibration that it is being used to measure. In many cases, this is very difficult to achieve. For example, sensors used in geophysical applications must accurately detect a wide range of possible forces. For accelerometers, this prior art approach to testing and calibration has the further serious disadvantage of requiring each accelerometer to be individually tested on a shaker or similar mechanical device. Further, shakers have been known to produce significant errors when the accelerometer under test is not directly in the main shaker axis.
There are in the prior art other self-testing concepts which have been proposed for use in sensors such as utilizing an integrated current loop for Hall effect magnetosensors which provide calibration through a fixed coupling. There have also been proposed precision capacitive structures for calibration. Implementing these concepts, however, requires complicated and highly precise mechanical and electronic structures with attendant manufacturing complexity and higher costs. Almost all accelerometers currently in the planning or production stage do not possess any self-testing functionality whatsoever.